High pressure combustion has gained favor because of the significant improvement in the overall thermal efficiency over what can be obtained from boilers operating at or near atmospheric pressure. The main reason for the increased thermal efficiency is the incorporation of a gas turbine in the cycle. High pressure combustion is most often associated with pressurized fluidized-bed combustion or coal gasification operating at pressures 10 to 20 times atmospheric pressure. Another advantage to high pressure combustion is that the equipment is smaller in size for a given energy output.
Fluidized-bed combustion offers versatility for burning a wide variety of fuels including many that are too poor in quality for use in conventional firing systems. Fuels which contain high concentrations of ash, sulfur and nitrogen can be burned efficiently while meeting stringent requirements for the control of stack emissions without the use of flue-gas scrubbers. Although liquid and gaseous fuels can be readily used in a high pressure fluidized-bed combustion system, the fuel is typically coal. It thus becomes apparent that a system must be provided to transport the solid fuel from atmospheric conditions and introduce it into the high pressure combustor. The same problem exists for the introduction of any solid sorbent which may be used, such as limestone or dolomite, for the capture of the sulfur oxides.
There are presently two types of systems in use for feeding solid fuel and/or sorbent material into high pressure combustors. The most common approach with solid fuels is to reduce the solids to fine particulates and then mix them with a liquid, such as oil or water, to form a pumpable slurry. The resulting slurry can then be pressurized and regulated using conventional slurry pumping equipment. The disadvantage is the need to add a liquid. The use of water reduces the efficiency of the combustion process because the water takes energy to vaporize. With oil, a more expensive fuel mixture is being used as opposed to using only coal. The other approach with dry solids is to employ a system of lockhoppers to pressurize the solids and then introduce them at elevated pressure into a high pressure pneumatic conveying system. See, for example, U.S. Pat. No. 4,335,733. Such a system normally does not have a very large turndown capability and requires the use of a considerable quantity of inert pressurizing gas for the lockhoppers.